This invention relates to a process for preparing certain 2-alkylthio substituted benzonitriles, which are useful as chemical intermediates, for example in the preparation of herbicidally active compounds.
2-Alkylthio-substituted benzonitriles are intermediates in the preparation of agrochemicals such as herbicides, for example as described in EP 0527036. It is desirable to provide such compounds in high yields and also to develop new procedures which allow the efficient displacement of 2-nitro or 2-halo substituted benzonitriles to furnish 2-alkylthio substituted benzonitriles.
The present invention seeks to provide a high yielding process for preparing 2-alkylthio substituted benzonitriles.
Thus, the present invention provides a process for preparing a 2-alkylthio-substituted-benzonitrile derivative of formula (I): 
wherein R1 represents C1-6 alkyl;
R2 represents C1-6 haloalkyl, C1-6 alkyl. C1-6 haloalkoxy. C1-6 alkoxy. SOnR5 or halogen;
R4 represents hydrogen, C1-6 haloalkoxy, C1-6 alkoxy. SOnR5 or halogen; or a 5 or 6-membered heterocyclic ring (which may be unsaturated or partially saturated) containing 1 to 3 hetero atoms selected from oxygen, nitrogen and sulphur, optionally substituted by halogen, C1-6 haloalkyl, C1-6 alkyl, C1-6 haloalkoxy, C1-6 alkoxy, SOnR5, nitro or cyano;
R5 represents C1-6 alkyl; and
n represents 0, 1 or 2; which comprises reacting a compound of formula (II): 
wherein R2 and R4 are as hereinbefore defined and R3 represents nitro or a halogen atom selected from fluorine, chlorine and bromine, with a compound of formula R1Sxe2x80x94X, wherein R1 is as hereinbefore defined and X is hydrogen or an alkali metal.
When R4 represents a heterocyclic ring, preferred rings include 3-isoxazolyl, 5-isoxazolyl, 2-thiazolyl, 5-oxazolyl, 2-furyl, 3-furyl, 2-thienyl and 3-thienyl.
R1 preferably represents methyl.
R2 preferably represents trifluoromethyl.
R3 preferably represents nitro or chlorine.
R4 preferably represents hydrogen.
X preferably represents a sodium, potassium or lithium atom.
In an especially preferred embodiment of the invention R1 represents C1-6 alkyl (methyl is most preferred);
R2 represents C1-6 haloalkyl (trifluoromethyl is most preferred);
R3 represents nitro or a halogen atom selected from fluorine, chlorine and bromine (nitro or chlorine are most preferred); and
R4 represents hydrogen.
The above reaction to prepare compounds of formula (I) by the reaction of a compound of formula (II) with a compound of formula R1Sxe2x80x94X may be performed using various solvents such as aromatic hydrocarbons for example toluene or xylene, ethers such as tetrahydrofuran, dioxan or tert-butyl methyl ether; amides such as N,N-dimethylformamide; sulphoxides such as dimethylsulphoxide; or ketones, for example methyl ethyl ketone or acetone. An especially preferred solvent is acetone, optionally in the presence of water. It has been found that the reaction proceeds in excellent yield using these conditions.
A preferred compound of formula R1Sxe2x80x94X is sodium thiomethoxide, which may be used in dry solid form or conveniently as a solution in water.
Where X is hydrogen, a base is generally present in the reaction mixture. Examples of suitable bases are alkali metal or alkaline earth metal carbonates, alkoxides or hydrides such as potassium carbonate, potassium t-butoxide or sodium hydride. or amidine bases such as 1,8-diazabicyclo[5.4.0]undec-7-ene or 1,1,3,3-tetramethylguanidine.
The reaction is generally performed at a temperature from about xe2x88x9220xc2x0 C. to about 120xc2x0 C. preferably from about 10xc2x0 to about 60xc2x0 C. and most preferably from about 10xc2x0 to about 40xc2x0 C.
The molar ratio of the benzonitrile derivative of formula (II): alkyl thiol (or metal salt thereof) of formula R1Sxe2x80x94X is generally from about 1:1 to about 1:4, preferably from about 1:1 to about 1:2.5 and most preferably from about 1:1 to about 1:1.2.
Optionally the reaction may be performed in a two phase system consisting of water and another solvent which has low solubility in water, in the presence a phase transfer catalyst. Examples of phase transfer catalysts which are suitable include ammonium salts such as tetrabutylammonium chloride; phosphonium salts such as tributylhexadecylphosphonium bromide; guanidinium salts such as hexaethylguanidinium chloride or hexamethylguanidinium chloride; or crown ethers such as 18-crown-6. Suitable solvents for use with water and the phase transfer catalyst include aromatic hydrocarbons for example toluene or xylene, ethers such as tert-butyl methyl ether, halogenated solvents such as chlorobenzene or dichloromethane, generally employed in admixture with water. The quantity of phase transfer catalyst employed is generally from a 2 to 10% molar ratio (relative to the molar amount of compound of formula (II)). When conducted under these conditions the reaction is generally carried out at a temperature of from about 5xc2x0 C. to about 100xc2x0 C., preferably from about 25xc2x0 to about 70xc2x0 C.
The following non-limiting examples illustrate the invention.